1. Field of the Invention
The invention generally relates to a gas delivery system. More particularly, the invention relates to a gas delivery system having one or more vaporizers that provide process gases on demand for substrate processing systems.
2. Background of the Related Art
In the production of integrated circuits, many processing methods require one or more reactive chemicals or precursors to be deposited onto a substrate in an atmospherically-controlled heated reactor or chamber. The precursors typically are converted from a solid or liquid state into a gaseous or vapor state to achieve a high degree of uniformity by vapor deposition. The precursor vapor, once generated, is directed into a reaction chamber and forms a deposited layer on the substrate. This process is typically called chemical vapor deposition or “CVD”. The deposited precursor chemical may form fine crystalline or amorphous layers which are required for creating microcircuits on the substrate.
In CVD processing systems, liquid precursors are typically delivered through a liquid flow meter to a vaporizer or bubbler which heats the liquid precursor into a vapor phase. The liquid precursors may be combined with a solvent to enhance the vaporization process. A carrier gas is also introduced into the vaporizer for carrying vaporized precursor molecules in the vapor phase to the processing chamber. The quantity and concentration of precursor introduced into the chamber is dependent on the flow of the carrier gas as well as the amount of precursor introduced into the vaporizer.
PLIS (precision liquid injection system), EPLIS and Parallel GPLIS have been developed to deliver vaporized liquid precursors to dielectric deposition chambers for deposition processes utilizing multiple liquid precursors, such as BPSG (borophosphosilicate glass), PSG (phosphosilicate glass or phosphorus-doped silicon oxide film), BSG (borosilicate glass or boron-doped silicon oxide film) or USG (undoped silicate glass or undoped silicon oxide film) processes.
Typically, the flow of the liquid precursor into the vaporizer is controlled by a liquid flow meter (LFM). The response time of the vapor supply into the chamber typically depends on the LFM PID (proportional-integral-differential) control, the liquid vaporizer control valve (injection valve) set up, liquid flow rate, liquid supply pressure, carrier gas flow rate, chamber pressure and etc. For a properly tuned liquid injection system, the response time before stable process gas flow in the chamber is reached typically ranges from about six to ten seconds.
FIG. 1 is a graphical illustration showing the standard flow response of vaporized liquid of a typical liquid injection system. The transient state due to the inherent rise time effect of the LFM is indicated by rise time, before liquid stabilizes to set point flow varies from liquid to liquid and from chamber to chamber. The transient film property at the film interface where film starts to grow can not be controlled and results in uncontrolled and inconsistent dopant concentration.
One example of a problem due to transient film properties is formation of voids at the interface of a BPSG layer and a nitride layer. Another example of a problem due to transient film properties is the consumption of nitride during anneal steps which occurs when a high phosphor content in the initial BPSG film in reaction with water vapors from a steam anneal process causes consumption of nitride by phosphoric acid. Inconsistent dopant concentration, particularly at interfaces with other materials, results in inconsistent processing and defective device formations.
This transient film property becomes even more significant when the deposition process is short as compared to the time for stabilizing process gas flow. For example, if a process requires a dopant to be introduced for about 12 seconds into the chamber, a majority of the doping process (i.e., about 6-10 seconds) will be required to stabilize the process gas flow, which may vary each time the process is performed, resulting in inconsistent and unrepeatable processing.
Therefore, there is a need for a process gas delivery system that improves dopant concentration control, particularly at film interfaces. More specifically, there is a need for accurate control of a vaporized liquid supply.